Scale UK Microplex to mp-300k-uk and derive smaller tiers

[MaxGhenis]

Umbrella: #2

After mp-30k-uk exists as a baseline candidate, build the larger UK Microplex frontier tier.

Scope:

• Construct an mp-300k-uk candidate using the largest defensible FRS/SPI/WAS/LCFS-backed universe available.
• Use L0 sparsity and post-fit pruning to derive smaller tiers where possible, rather than relying permanently on up-front sampling.
• Add local/geographic validation where targets exist.
• Profile stages and add disk/runtime guardrails before treating the tier as routinely runnable.

Acceptance:

• mp-300k-uk either improves the quality/runtime frontier or is explicitly marked experimental in the dashboard with reproducible loss, runtime, and sparsity metrics.